Magnetic electron lens



1953 J. A. V-ERHOEF ETAL 2,849,636

MAGNETIC ELECTRON LENS Filed Nov. 28, 1952 ADJUSTABLE MEANS IN VEN TORSJOHAN ANTON VERHOEF WILHELM GODFRIED AMESZ AGENT United States PatentMAGNETIC ELECTRON LENS Johan Anton Verhoef and Wilhelm Godfried Amesz,

Eindhoven, Netherlands, assignors, by mesne assignments, to NorthAmerican Philips Company, Inc., New York, N. Y., a corporation ofDelaware Application November 28, 1952, Serial No. 322,962 Claimspriority, application Netherlands January 4, 1952 8 Claims. (Cl. 313-84)This invention relates to magnetic electron lenses with adjustable focallength such as are used, for example, for focusing the electron beam incathode-ray tubes, for example of television receivers.

In a known device of this type, use is made of a permanent magnet havingrotational symmetry and being magnetized in an axial direction, thefocal length of the lens being variable by means of a movablecylindrical ferro-magnetic shunt embracing the permanent magnet.

Such electron lenses suffer from the disadvantage that the stray orleakage field produced outside the permanent magnet is unduly large.Notably in cathode-ray tubes for television receivers, said leakagefield has been found to extend to the deflection coil or to the iontrap, which involves undue distortion of the deflection of the beam andcauses the ion trap to cease to function properly.

The present invention has for its object a magnetic electron lens ofvariable focal length with which said stray field is avoided in a simplemanner. It is characterized by the use of two, co-axial, disc-shaped,permanent magnets which are movable with respect to each other and thefields of which substantially neutralize each other on the axis outsidethe magnets. According to a further aspect of the invention, saidneutralization is achieved in a simple manner by opposite directions ofmagnetisation of both magnets.

In order that the invention may be readily carried into eflect, it willnow be described with reference to the accompanying drawing representingembodiments thereof and in which:

Fig. 1 illustrates the principle of a magnetic electron lens in whichthe permanent magnets are magnetized radially;

Fig. 2 shows the principle of a magnetic electron lens in which thepermanent magnets are magnetized axially;

Fig. 3 is a cross-sectional view of the neck area of a cathode-ray tubein a television receiver and containing a focusing, magnetic lens of theinvention.

In the three figures, corresponding elements are provided with the samereference numerals.

The magnetic electron lens shown in Fig. 1 comprises two, annular,disc-shaped, co-axial, permanent magnets 1 and 2, Whose directions ofmagnetisation NS extend radially i. e. for the magnet 1 from the insideto the outside, and for the magnet 2 from the outside to the inside. Inthis manner, a radially symmetric electron-focusing magnetic field isproduced about the axis of symmetry 3 of the magnets 1 and 2, the valueH, of said active magnetic field between the magnets being fairly high.However, the stray field H outside the magnets is feeble, since theoutside fields produced by each individual magnet 1 and 2 substantiallyneutralize each other in situ. In order that the field H at the outerperiphery of the magnets shall not extend too far, a ferromagnetic, forexample soft iron, shielding box 4 may be provided. As will be noted,however, the space between the magnets 1 and 2 is free of otherferromagnetic members.

The focal length of the electron lens is adjustable by 2,849,636Patented Aug. 26, 1958 making, for example, the magnet 2 stationary andby shifting the other magnet 1 in an axial direction by means of adriving mechanism 5, the co-axial position of the magnets 1 and 2 beingmaintained by means of a guide (not shown). In this instance compressionsprings are unnecessary, since the magnets 1 and 2 attract each othercontinuously.

Fig. 2 shows a similar electron lens, but in this instance the annularpermanent magnets 1 and 2 are magnetized axially with oppositedirections of magnetisation NS. Such a lens has the advantage over thatshown in Fig. 1 that the active field H along the axis at the magnet 1is as large and oppositely directed compared with that at the magnet 2resulting in avoidance of image rotation. Further, since the magnetsrepel one another, the focuschanging mechanism 5 is now required toengage that side of the magnet 1 which faces away from the stationarymagnet 2, thus simplifying the construction.

In order that centering of the image produced may be controlled, a plate6 movable in a radial direction may be provided in the proximity of theside of the magnet 1 facing the source of electrons, said plate causingdissymmetry of the magnet field with consequent image displacement.

Notably with the electron lens shown in Fig. 2, where the direction ofmagnetisation NS coincides with the direction of thickness of themagnets 1, 2 and, moreover these magnets are influenced by theirrespective demagnetizing fields, the permanent magnetic materialpreferably has a remanent induction B which, measured in gauss, issmaller than four times the coercive field strength H measured inoersteds, since in spite of the demagnetizing fields the magnets retaintheir magnetic properties to a sufiicient degree. Materials satisfyingthis condition and suitable for application in the invention are thepermanent magnet materials which are fully described in United StatesPatent No. 2,762,777. These materials are characterized by a compositionsubstantially consisting of non-cubic crystals consisting principally ofa composite oxide of iron and of at least one of the metals barium,

strontium and lead, and, alternatively, a small amount of calcium as apartial substituent for one of the latter metals.

Fig. 3 shows the lens of Fig. 2 mounted in position on the neck of acathode-ray tube 10 adapted for use in a television receiver. Only theneck portion of the cathoderay tube 10 is shown, the electron-receiving,phosphor screen (not shown) being located at the right. The tubecontains the usual bent, electron gun 11 for producing an electron beamfree of ions along the axis 3 of the tube and the lens. This end isattained in cooperation with an ion trap 12 mounted on the neck andproducing within the tube a magnetic field. Closer to the phosphorscreen are the usual deflection coils 13, which establish within thetube a magnetic field causing the beam to scan a raster on the phosphorscreen. The focusing unit for the beam, which is magnetic in this case,is mounted between the ion trap and deflection coils. Due to thenearness of the latter, as explained above, the stray or leakage fieldof the focusing unit should be a minimum, especially along the axis 3,to avoid interference with the ion trap and deflection fields. Thiselfect is obtained with the lens of the invention, as the opposedmagnetization of the two magnets tends to neutralize the axial,external, stray fields. To vary the lens power or its focal length, theaxial spacing between the magnets may be varied. As shown, this isobtained by making the magnet 2 stationary, and providing means 5 foraxially displacing the magnet 1 relative thereto.

What we claim is:

1. In combination, means producing a charged particle beam along a givenpath and means for focusing the beam, said focusing means comprising apair of substantially annular, permanent magnets mounted substantiallycoaxially with and adjacent one another and surrounding the beam-pathand cooperating to produce an active magnetic field substantially attheir axes and substantially in the area within the magnets, saidmagnets being magnetized' in opposite directions whereby the straymagnetic fields. produced'by said pair of magnets attheir axes on theside of each magnet remote from the other magnet are substantiallyneutralized, and means for varying the axial. spacing between themagnets thereby to vary the strength of the activemagnetic field and thefocusing etiect on the beam.

2. A variable magnetic electron lens adapted for focusing' an electronbeam passing therethrough, comprising two, flat, ring-shaped, permanentmagnets mounted adjacent and coaxial with one another, said permanentmagnets definingtherebetween an area free of other ferromagnetic.material and defining along the axes of and within the coaxialmagnets anempty area to be traversed by the electron beam and within which anactive magnetic field is. established by said cooperating magnets tofocus said beam, said magnets being magnetized in opposite directionswhereby the stray magnetic fields produced on the axes but outside themagnets are reduced to a minimum, and means for varying the axialspacing of the magnets thereby to vary the strength of the activemagnetic field and the focusing effect on an electron beam.

3. A variable magnetic electron lens as set forth in claim 2, wherein.the permanent magnets are each constitutedof a ferromagnetic materialhaving a ratio of remanence in gauss to coercive field strength H inoersteds of lessthan 4.

4. A. variable magnetic electron lens as set forth in claim 3. whereinthe ferromagnetic material consists prin- .cipally of a'composite oxideof iron and a metal selected from the group consisting of barium,strontium and lead.

5. A variable permanent magnet focusing device comprising a pair ofcoaxially-mounted, adjacent, disc-shaped, annular, permanent magnets,said magnets being magnetizedin a direction parallel to their axes butin opposite directions, thereby producing poles of the same polarity '4on facing surfaces of said magnets, and means for varying axially thespacing between the magnets thereby to vary the strength of a magneticfield produced by said device.

6. In combination, means producing a charged particle beam along a givenpath and means for focusing said beam, said focusing means comprising apair of coaxiallymounted, adjacent, disc-shaped, permanent magnets,surrounding the beam path, said magnets being magnetized in a directionat right angles to their axes but in opposite directions, therebyproducing poles of opposite polarity on the outer peripheral surfaces ofsaid magnets, and means for varying axially the spacing between themagnets thereby to vary the strength of the magnetic field produced bysaid focusing means at the beam path and thus vary the focusing effecton the beam.

In combination, means producing an electron beam along a given path andmeans for focusing said beam, said focusing means comprising a pair ofcoaxially-mounted, adjacent, disc-shaped, permanent magnets surroundingthe electron beam path, said permanent magnets being magnetized inopposite directions and establishing a strong, active, magnetic field atthe beam path within the magnets but a weak stray field at the beam pathoutside of the magnets, thereby to limit the active magnetic field ofthe focusing means to their immediate vicinity, said focusing meansincluding means for varying the spacing between the magnets by movingthe magnets relative to one another thereby tovary the focusing of theelectron beam.

8. A combination as set forth in claim 1, wherein means are provided forcentering the charged particle beam, said centering-means comprising aradially-movable plate-like member mounted adjacent the side of one ofthe magnets remote from the other magnet.

References Cited in the file of this patent UNITED STATES PATENTS2,582,788 Mendelsohn Jan. 15, 1952 2,615,738 Johnson Oct. 28, 19522,678,729 Spodig May 18, 1954 2,714,678 Wolff Aug. 2, 1955 2,722,617Cluwen et a1. Nov. 1, 1955 2,799,813 Rademakers et a1. July 16, 1957

